In the "Doctor Who" 60th Anniversary Special “The Giggle,” a unique regeneration concept called "bi-generation" is introduced, featuring both the 14th and 15th Doctors.
David Tennant’s 14th Doctor uses bi-generation to regenerate simultaneously into two distinct entities, leading to the emergence of the 15th Doctor portrayed by Ncuti Gatwa.
Fourteen will now live a normal life and heal from a millennium of trauma until he inevitably regenerates into Fifteen, creating a temporal loop where Fifteen is Fourteen’s future.
This mirrors showrunner Russell T. Davies’ earlier storytelling twists, as when the Doctor retained his regeneration by transferring energy into a severed hand and cloning himself.
Davies’ first two specials show the Doctor’s need for a health break, neatly wrapping up the current Doctor’s story and setting the stage for the 15th Doctor’s healthy beginning.